Cordierite aluminum magnesium titanate compositions and ceramic articles comprising same

ABSTRACT

Disclosed are ceramic bodies comprised of composite cordierite aluminum magnesium titanate ceramic compositions and methods for the manufacture of same.

This application is a divisional of and claims the benefit of U.S.patent application Ser. No. 13/690,096, filed Nov. 30, 2012, which ishereby incorporated by reference for all purposes as if fully set forthherein.

BACKGROUND

Field

Exemplary embodiments of the present disclosure relate to ceramiccompositions and to composite ceramic compositions comprised ofcordierite aluminum magnesium titanate.

Discussion of the Background

Refractory materials with low thermal expansion, and consequently highthermal shock resistance, are used in applications such as catalyticconverter substrates and diesel particulate filters where high thermalgradients exist during use. A material for these applications iscordierite due to its low thermal expansion, high melting point, and lowcost. In the diesel particulate filter area, it has been recognized thathigher heat capacity is desirable for improving durability of filtersduring regeneration. A material with a high volumetric heat capacitylowers the volume of material necessary to absorb a given amount ofheat. Less material volume may reduce pressure drop in the exhauststream and increase the open volume for ash storage. However, lowthermal expansion is still desired. Aluminum titanate is a material thatcan be made with low thermal expansion and also has higher volumetricheat capacity than cordierite.

Pure aluminum titanate is metastable below about 1250° C. The thermalexpansion of AT is low when the grain size is large and microcracks formduring cooling after firing. These large grains and microcracks tend tomake the material mechanically weak. As a consequence of themicrocracks, the thermal expansion curve can have large hysteresis,leading to high values of instantaneous thermal expansion, especially oncooling. The firing temperature of AT-based composites is typicallyhigh, usually above 1400° C. Finally, AT has been shown to exhibit highthermal cycling growth which can be exaggerated by the presence ofalkali elements.

To slow down the decomposition rate, additives such as mullite, MgTi₂O₅,and Fe₂TiO₅ may be added to the aluminum titanate. MgTi₂O₅ tends to slowthe decomposition rate in reducing conditions and only slows the rate inoxidizing conditions at high levels (>10%). Fe₂TiO₅ tends to slow thedecomposition rate in oxidizing conditions and increase thedecomposition rate in reducing conditions.

Second phases such as mullite have been added to AT to increase thestrength of the composite body because microcracking generally does notoccur between mullite crystals. Mullite also has a fairly highvolumetric heat capacity. Other second phases have also been used in ATcomposites, including alkali and alkaline earth feldspars. However,mullite and alkali feldspars have a higher than optimum thermalexpansion.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the disclosure andtherefore it may contain information that does not form any part of theprior art nor what the prior art may suggest to a person of ordinaryskill in the art.

SUMMARY

Exemplary embodiments of the present disclosure provide compositeceramic compositions comprising cordierite-pseudobrookite.

Exemplary embodiments of the present disclosure also provide a dieselparticulate filter comprised of a composite composition of cordieritealuminum magnesium titanate.

Exemplary embodiments of the present disclosure also provide a methodfor manufacturing a composite cordierite aluminum magnesium titanateceramic article.

Additional features of the claimed invention will be set forth in thedescription which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention as claimed.

An exemplary embodiment discloses a ceramic article comprising apseudobrookite phase comprising predominately alumina, magnesia, andtitania; a second phase comprising cordierite; and a sintering aid,comprising at least one of a calcium oxide and a strontium oxide.

An exemplary embodiment discloses an article comprising a compositecomposition of a solid solution of aluminum titanate and magnesiumdititanate and a second crystalline phase comprising cordierite. Thearticle has a composition, as expressed in weight percent on an oxidebasis of from 4 to 10% MgO; from 40 to 55% Al₂O₃; from 25 to 44% TiO₂;from 5 to 25% SiO₂, and a sintering aid, the sintering aid including atleast one of a calcium oxide and a strontium oxide.

An exemplary embodiment also discloses a diesel particulate filtercomprised of a composite composition of a solid solution of aluminumtitanate and magnesium dititanate and a second crystalline phasecomprising cordierite. The particulate filter has a composition, asexpressed in weight percent on an oxide basis of from 4 to 10% MgO; from40 to 55% Al₂O₃; from 25 to 44% TiO₂; from 5 to 25% SiO₂, and asintering aid, the sintering aid including at least one of a calciumoxide and a strontium oxide. In an exemplary embodiment the dieselparticulate filter comprises a honeycomb structure having a plurality ofaxially extending end-plugged inlet and outlet cells.

An exemplary embodiment also discloses a method for manufacturing acomposite cordierite aluminum magnesium titanate ceramic article. Themethod includes compounding an inorganic batch composition comprising amagnesia source, a silica source, an alumina source, a titania source,and at least one sintering aid, wherein the sintering aid includes atleast one of a calcium oxide and a strontium oxide. Mixing the inorganicbatch composition together with one or more processing aid selected fromthe group consisting of a plasticizer, lubricant, binder, pore former,and solvent, to form a plasticized ceramic precursor batch composition.Shaping the plasticized ceramic precursor batch composition into a greenbody. The method includes firing the green body under conditionseffective to convert the green body into a ceramic article comprising afirst crystalline phase comprised predominantly of a solid solution ofaluminum titanate and magnesium dititanate and a second crystallinephase comprising cordierite.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings, which are included to provide a furtherunderstanding of the disclosure and are incorporated in and constitute apart of this specification, illustrate exemplary embodiments of theclaimed invention, and together with the description serve to explainthe principles of the invention as claimed.

FIG. 1 depicts the approximate stable combination of phases as afunction of temperature and composition along the pseudo-binary joinbetween aluminum titanate (Al₂TiO₅) and cordierite (Mg₂Al₄Si₅O₁₈).

FIGS. 2A and 2B depict the approximate phase relations at 1325° C. inthe pseudo-ternary sections with endpoints of magnesium dititanate,aluminum titanate, and cordierite within the quaternaryMgO—Al₂O₃—TiO₂—SiO₂ system.

FIG. 3 illustrates the change in length as a function of time at 1100°C. for a control aluminum titanate ceramic composition and for acomposition in the cordierite/mullite/pseudobrookite region of the phasediagram.

FIG. 4 demonstrates the change in the 25-1000° C. coefficient of thermalexpansion for a control aluminum titanate ceramic composition and thecordierite/mullite/pseudobrookite composition of Table 1 after 100 hoursat temperatures of from 950 to 1250° C.

FIG. 5 shows representative data for pressure drop as a function of sootloading for a cordierite/mullite/pseudobrookite ceramic wall flow filtermade in accordance with an exemplary embodiment of the disclosure.

FIG. 6 depicts the microstructure of an exemplary embodiment of adisclosed body with approximately 55 grams/liter of alumina washcoat.

FIG. 7 shows the coefficient of thermal expansion (CTE) as a function ofrelative rare earth cost (1% Y₂O₃=1) for exemplary embodiments of thedisclosure.

FIG. 8 shows schematic exemplary embodiment of a time-temperature (t-T)graph 80 illustrating top (first hold) temperature 82, low (second hold)temperature 84, and mid (third hold) temperature 86.

DETAILED DESCRIPTION

It will be understood that for the purposes of this disclosure, “atleast one of X, Y, and Z” can be construed as X only, Y only, Z only, orany combination of two or more items X, Y, and Z (e.g., XYZ, XYY, YZ,ZZ).

In an effort to provide a composite AT ceramic body having improvedstrength while maintaining a low CTE, cordierite would be a betterchoice than mullite as a second phase because cordierite has a lowercoefficient of thermal expansion than does mullite. However, cordieriteand pure aluminum titanate are not in thermodynamic equilibrium at anytemperature. The provision of a cordierite and AT based compositeceramic having low CTE, high strength, and good thermal stabilityrepresents an advancement in the state of the art.

An exemplary embodiment of the present disclosure provides a compositeceramic body comprising a first crystalline phase comprisedpredominantly of a solid solution of aluminum titanate and magnesiumdititanate (MgTi₂O₅—Al₂TiO₅) and a second crystalline phase comprisingcordierite. The compositions of the ceramic bodies can be characterizedas comprising, when expressed on weight percent oxide basis: from 4 to10% MgO; from 40 to 55% Al₂O₃; from 25 to 42% TiO₂; from 5 to 25% SiO₂,from 0 to 5% CeO₂, and at least one of from 0.15 to 1% CaO and from 0.1to 2% SrO. In these or other exemplary embodiments, the compositions ofthe ceramic bodies of the disclosure are expressed in terms of weightfractions of oxides and oxide combinations to comprise, on an oxidebasis,a(Al₂TiO₅)+b(MgTi₂O₅)+c(2MgO.2Al₂O₃.5SiO₂)+d(3Al₂O₃.2SiO₂)+e(MgO.Al₂O₃)+f(2MgO.TiO₂)+g(CaO)+h(SrO)+i(X)+j(Fe₂O₃.TiO₂)+k(TiO₂)+l(Al₂O₃),wherein X can be at least one of CeO₂, Y₂O₃, and La₂O₃, and a, b, c, d,e, f, g, h, i, j, k, and l are weight fractions of each component suchthat (a+b+c+d+e+f+g+h+i+j+k+l)=1.00. To that end, the weight fraction ofeach component can be in the respective ranges as follows: 0.3≤a≤0.75,0.075≤b≤0.3, 0.02≤c≤0.5, 0.0≤d≤0.4, 0.0≤e≤0.25, 0.0≤f≤0.1, 0.0≤g≤0.01,0.0≤h≤0.02, 0.0015≤(g+h), 0.0≤i≤0.05, 0.0≤j≤0.05, 0.0≤k≤0.20, and0.0≤l≤0.10. It will be recognized that the oxides and oxide combinationsused to define the oxide compositions of these ceramics will notnecessarily be present in the ceramic bodies as the corresponding freeoxides or crystal phases, other than as those crystal phases arespecifically identified herein as characteristic of these ceramics. Itwill also be recognized that while the sum of a, b, c, d, e, f, g, h, i,j, k, and l is 1.00, it is the ratio of oxides and oxide combinationsthat are expressed. That is, the composite ceramic body may includeother impurities in addition to the ratio of oxides and oxidecombinations expressed. This will be apparent in view of the examplesdisclosed below.

The solid solution aluminum titanate and magnesium dititanate phasepreferably exhibits a pseudobrookite crystal structure. To that end, thecomposition of the pseudobrookite phase can depend upon the processingtemperature as well as the overall bulk composition of the ceramic and,as such, can be determined by an equilibrium condition. However, in anexemplary embodiment, the composition of the pseudobrookite phasecomprises from approximately 15% to 35% MgTi₂O₅ by weight. Stillfurther, while the total volume of the pseudobrookite phase can alsovary, in another exemplary embodiment, the total volume can be in therange of about 50 to 95 volume % of the overall ceramic composition.

Optionally, the composite ceramic body can further comprise one or morephases selected from the group consisting of mullite, sapphirine, atitania polymorph such as rutile or anatase, corundum, and a spinelsolid solution (MgAl₂O₄-Mg₂TIO₄). When present, the composition of thespinel phase will also depend on processing temperatures and overallbulk composition. However, in an exemplary embodiment, the spinel phasecan comprise at least about 95% MgAl₂O₄.

Still further, the ceramic composition can also comprise one or moresintering aid(s) or additives provided to lower the firing temperatureand broaden the firing window required to form the ceramic composition.A sintering aid can, for example, be present in an amount of from 0.15to 5 weight percent of the total composition and can include, forexample, one or more metal oxides such as CaO, SrO, CeO₂, Y₂O₃, andLa₂O₃.

In an exemplary embodiment, yttrium oxide (Y₂O₃) and/or lanthanum oxide(La₂O₃) has been found to be a particularly good sintering additive whenadded in an amount of between 0.5 and 4.0 wt. %, for example, between1.0 and 2.0 wt. %. To that end, the yttrium oxide or lanthanide oxidemay be present as the oxide phase, or may form a new phase with one ormore of the other metal oxide constituents of the ceramic body.Similarly, iron oxide from a suitable iron source, present as ferrous orferric oxide or in combination with other oxides, e.g., as Fe₂TiO₅, canbe present in some embodiments in an amount, calculated as Fe₂TiO₅, offrom 0 to 3 weight % Fe₂TiO₅. The presence of Fe₂TiO₅ can be useful forslowing decomposition in oxidizing atmospheres. When both Fe₂TiO₅ and aspinel phase are present in the ceramic body, the spinel solid solutioncan also additionally contain ferrous and/or ferric iron in the solidsolution. Furthermore, the sintering aid can include cerium oxide (CeO₂)or cerium oxide in combination with one or more other metal oxides suchas Y₂O₃, and La₂O₃. For example, the sintering aid can include ceriumoxide in combination with yttrium oxide, cerium oxide in combinationwith lanthanum oxide, or cerium oxide in combination with yttrium oxideand lanthanum oxide.

In U.S. patent application Ser. No. 12/305,767, the entire contents ofwhich are herein incorporated by reference, cordierite, mullite,pseudobrookite composites with high porosity and low thermal expansionare described having a wide firing window when yttrium oxide is added tothe batch. The present application recognizes a need for a wide firingwindow while avoiding the high cost of yttrium oxide and other rareearth elements.

According to an exemplary embodiment of the present disclosure thesintering aid can include calcium oxide (CaO), strontium oxide (SrO),calcium oxide in combination with strontium oxide, calcium oxide incombination with one or more other metal oxides such as cerium oxide,yttrium oxide (Y₂O₃), and lanthanum oxide (La₂O₃), strontium oxide incombination with one or more other metal oxides such as cerium oxide,Y₂O₃, and La₂O₃, or calcium oxide and strontium oxide in combinationwith one or more other metal oxides such as cerium oxide, Y₂O₃, andLa₂O₃. For example, the sintering aid can include calcium oxide, calciumoxide in combination with yttrium oxide, calcium oxide in combinationwith lanthanum oxide, calcium oxide in combination with cerium oxide,calcium oxide in combination with yttrium oxide and lanthanum oxide,calcium oxide in combination with yttrium oxide and cerium oxide,calcium oxide in combination with cerium oxide and lanthanum oxide, orcalcium oxide in combination with yttrium oxide, lanthanum oxide, andcerium oxide. For example, the sintering aid can include strontiumoxide, strontium oxide in combination with yttrium oxide, strontiumoxide in combination with lanthanum oxide, strontium oxide incombination with cerium oxide, strontium oxide in combination withyttrium oxide and lanthanum oxide, strontium oxide in combination withyttrium oxide and cerium oxide, strontium oxide in combination withcerium oxide and lanthanum oxide, or strontium oxide in combination withyttrium oxide, lanthanum oxide, and cerium oxide. For example, thesintering aid can include calcium oxide and strontium oxide, calciumoxide and strontium oxide in combination with yttrium oxide, calciumoxide and strontium oxide in combination with lanthanum oxide, calciumoxide and strontium oxide in combination with cerium oxide, calciumoxide and strontium oxide in combination with yttrium oxide andlanthanum oxide, calcium oxide and strontium oxide in combination withyttrium oxide and cerium oxide, calcium oxide and strontium oxide incombination with cerium oxide and lanthanum oxide, or calcium oxide andstrontium oxide in combination with yttrium oxide, lanthanum oxide, andcerium oxide. That is, the sintering aid can include, for example, atleast one of CaO and SrO in combination with at least one of Y₂O₃, CeO₂,and La₂O₃.

The inventors have found that calcium oxide, strontium oxide, ormixtures of calcium oxide and strontium oxide with one or more othermetal oxides such as cerium oxide, Fe₂TiO₅, yttrium oxide, and lanthanumoxide, result in similar CTE, porosity, pore size, and pore sizedistribution at lower rare earth cost than yttrium oxide alone oryttrium oxide with other rare earth elements.

In an exemplary embodiment the amount of calcium oxide can be in a rangeof about 0.15 to about 1.0 wt % and/or the amount of strontium oxide canbe in a range of about 0.1 to about 2.0 wt %. For example, the amount ofcalcium oxide can be in a range of 0.2 to 0.9 wt %, 0.25 to 0.75 wt %,and 0.4 to 0.6 wt %. For example, the amount of strontium oxide can bein a range of 0.16 to 1.8 wt %, 0.2 to 1.6 wt %, and 0.3 to 1.5 wt %.

As mentioned, in an exemplary embodiment the mixtures of calcium oxideand/or strontium oxide with one or more other metal oxides such ascerium oxide, yttrium oxide, and lanthanum oxide can be a sintering aid.The amount of the mixture can be in a range of 0.15 to 5.0 wt %. Forexample, the amount of the mixture can be in a range of 0.3 to 3.0 wt %,0.4 to 2.5 wt %, 0.5 to 1.5 wt %, and 2.5 to 4.5 wt %.

According to an exemplary embodiment of the present disclosure, theceramic body comprises approximately 10 to 25 wt % cordierite,approximately 5 to 30 wt % mullite, approximately 50 to 70 wt % of apseudobrookite phase consisting predominantly of an Al₂TiO₅—MgTi₂O₅solid solution, and approximately 0.15 to 3.0 wt % of at least one ofCaO and SrO addition. According to another exemplary embodiment of thepresent disclosure, the ceramic body comprises approximately 10 to 25 wt% cordierite, approximately 5 to 30 wt % mullite, approximately 50 to 70wt % of a pseudobrookite phase consisting predominantly of anAl₂TiO₅—MgTi₂O₅ solid solution, approximately 0.1 to 3.0 wt % CeO₂addition, and an addition of at least one of CaO at approximately 0.15to 1.0 wt % and SrO at approximately 0.1 to 2.0 wt %.

Exemplary embodiments of the ceramic bodies of the present disclosurecan in some instances comprise a relatively high level of totalporosity. For example, bodies comprising a total porosity, % P, of atleast 40%, at least 45%, at least 50%, or even at least 60%, asdetermined by mercury porosimetry, can be provided.

In addition to the relatively high total porosities, ceramic bodies ofthe present disclosure can also comprise a relatively narrow pore sizedistribution evidenced by a minimized percentage of relatively fineand/or relatively large pore sizes. To this end, relative pore sizedistributions can be expressed by a pore fraction which, as used herein,is the percent by volume of porosity, as measured by mercuryporosimetry, divided by 100. For example, the quantity d₅₀ representsthe median pore size based upon pore volume, and is measured inmicrometers; thus, d₅₀ is the pore diameter at which 50% of the openporosity of the ceramic sample has been intruded by mercury. Thequantity d₉₀ is the pore diameter at which 90% of the pore volume iscomprised of pores whose diameters are smaller than the value of d₉₀;thus, d₉₀ is also equal to the pore diameter at which 10% by volume ofthe open porosity of the ceramic has been intruded by mercury. Stillfurther, the quantity d₁₀ is the pore diameter at which 10% of the porevolume is comprised of pores whose diameters are smaller than the valueof d₁₀; thus, d₁₀ is equal to the pore diameter at which 90% by volumeof the open porosity of the ceramic has been intruded by mercury. Thevalues of d₁₀ and d₉₀ are also expressed in units of micrometers.

The median pore diameter, d₅₀, of the pores present in the instantceramic articles can, in one embodiment, be at least 10 μm, morepreferably at least 14 μm, or still more preferably at least 16 μm. Inanother embodiment, the median pore diameter, d₅₀, of the pores presentin the instant ceramic articles do not exceed 30 μm, and more preferablydo not exceed 25 μm, and still more preferably do not exceed 20 μm. Instill another embodiment, the median pore diameter, d₅₀, of the porespresent in the instant ceramic articles can be in the range of from 10μm to 30 μm, more preferably from 18 μm to 25 μm, even more preferablyfrom 14 μm to 25 μm, and still more preferably from 16 μm to 20 μm. Tothis end, a combination of the aforementioned porosity values and medianpore diameter values can provide low clean and soot-loaded pressure dropwhile maintaining useful filtration efficiency when the ceramic bodiesof the present disclosure are used in diesel exhaust filtrationapplications.

The relatively narrow pore size distribution of the exemplaryembodiments of the ceramic articles can, in one embodiment, be evidencedby the width of the distribution of pore sizes finer than the medianpore size, d₅₀, further quantified as pore fraction. As used herein, thewidth of the distribution of pore sizes finer than the median pore size,d₅₀, are represented by a “d_(factor)” or “d_(f)” value which expressesthe quantity (d₅₀−d₁₀)/d₅₀. To this end, the ceramic bodies of thepresent disclosure can comprise a d_(factor) value that does not exceed0.50, 0.40, 0.35, or even that does not exceed 0.30. In some exemplaryembodiments, the d_(factor) value of the disclosed ceramic body does notexceed 0.25 or even 0.20. To this end, a relatively low d_(f) valueindicates a low fraction of fine pores, and low values of d_(f) can bebeneficial for ensuring low soot-loaded pressure drop when the ceramicbodies are utilized in diesel filtration applications.

The relatively narrow pore size distribution of the disclosed ceramicarticles can in another exemplary embodiment also be evidenced by thewidth of the distribution of pore sizes that are finer or coarser thanthe median pore size, d₅₀, further quantified as a pore fraction. Asused herein, the width of the distribution of pore sizes that are fineror coarser than the median pore size, d₅₀, are represented by a“d_(breadth)” or “d_(B)” value which expresses the quantity(d₉₀−d₁₀)/d₅₀. To this end, the ceramic structure of the presentdisclosure in one exemplary embodiment comprises a d_(b) value that isless than 1.50, less than 1.25, less than 1.10, or even less than 1.00.In some exemplary embodiments, the value of d_(b) is not more than 0.8,more preferably not greater than 0.7, and even more preferably notgreater than 0.6. A relatively low value of d_(b) can provide arelatively higher filtration efficiency and higher strength for dieselfiltration applications.

Another exemplary embodiment of the ceramic bodies exhibit a lowcoefficient of thermal expansion resulting in excellent thermal shockresistance (TSR). As will be appreciated by one of ordinary skill in theart, TSR is inversely proportional to the coefficient of thermalexpansion (CTE). That is, a ceramic body with low thermal expansion willtypically have higher thermal shock resistance and can survive the widetemperature fluctuations that are encountered in, for example, dieselexhaust filtration applications. Accordingly, in one exemplaryembodiment, the ceramic articles of the present disclosure arecharacterized by having a relatively low coefficient of thermalexpansion (CTE) in at least one direction and as measured bydilatometry, that is less than or equal to about 25.0×10⁻⁷/° C., lessthan or equal to 20.0×10⁻⁷/° C.; less than or equal to 15.0×10⁻⁷/° C.,less than or equal to 10.0×10⁻⁷/° C., or even less than or equal to8.0×10⁻⁷/° C., across the temperature range of from 25° C. to 1000° C.

Still further, it should be understood that exemplary embodiments canexhibit any desired combination of the aforementioned properties. Forexample, in one embodiment, it is preferred that the CTE (25-1000° C.)does not exceed 12×10⁻⁷/° C. (and preferably not more than 10×10⁻⁷/°C.), the porosity % P is at least 45%, the median pore diameter is atleast 14 μm (and preferably at least 18 μm), and the value of d_(f) isnot more than 0.35 (and preferably not more than 0.30). It is furtherpreferred that such exemplary ceramic bodies exhibit a value of d_(b)that does not exceed 1.0, and more preferably that does not exceed 0.85,and still more preferably that does not exceed 0.75. In anotherexemplary embodiment, the CTE (25-1000° C.) does not exceed 18×10⁻⁷/° C.and the porosity % P is at least 40%. For example, the CTE (25-1000° C.)does not exceed 18×10⁻⁷/° C. and the porosity % P is at least 60%. Inanother example, CTE (25-1000° C.) does not exceed 12×10⁻⁷/° C. and theporosity % P is at least 40%. In a further example, CTE (25-1000° C.)does not exceed 12×10⁻⁷/° C. and the porosity % P is at least 60%.

The ceramic bodies of the present disclosure can have any shape orgeometry suitable for a particular application. In high temperaturefiltration applications, such as diesel particulate filtration, forwhich the ceramic bodies are especially suited, it is preferred thebodies to have a multicellular structure, such as that of a honeycombmonolith. For example, in an exemplary embodiment, the ceramic body cancomprise a honeycomb structure having an inlet and outlet end or face,and a multiplicity of cells extending from the inlet end to the outletend, the cells having porous walls. The honeycomb structure can furtherhave cellular densities from 70 cells/in² (10.9 cells/cm²) to 400cells/in² (62 cells/cm²). A portion of the cells at the inlet end orface end can, in one embodiment, be plugged with a paste having same orsimilar composition to that of the honeycomb structure, as described inU.S. Pat. No. 4,329,162 which is herein incorporated by reference. Theplugging is only at the ends of the cells which is typically to a depthof about 5 to 20 mm, although this can vary. A portion of the cells onthe outlet end but not corresponding to those on the inlet end areplugged. Therefore, each cell is plugged only at one end. A preferredarrangement is to have every other cell on a given face plugged as in acheckered pattern.

This plugging configuration allows for more intimate contact between theexhaust stream and the porous wall of the substrate. The exhaust streamflows into the substrate through the open cells at the inlet end, thenthrough the porous cell walls, and out of the structure through the opencells at the outlet end. Filters of the type herein described are knownas “wall flow” filters since the flow paths resulting from alternatechannel plugging require the exhaust being treated to flow through theporous ceramic cell walls prior to exiting the filter.

Exemplary embodiments of the present disclosure also provide a method ofmanufacturing composite cordierite aluminum magnesium titanate ceramicarticles from a ceramic forming precursor batch composition comprised ofcertain inorganic powdered raw materials. Generally, the method firstcomprises providing an inorganic batch composition comprising a magnesiasource, a silica source, an alumina source, and a titania source. Theinorganic batch composition is then mixed together with one or moreprocessing aid(s) selected from the group consisting of a plasticizer,lubricant, binder, pore former, and solvent, to form a plasticizedceramic precursor batch composition. The plasticized ceramic precursorbatch composition can be shaped or otherwise formed into a green body,optionally dried, and subsequently fired under conditions effective toconvert the green body into a ceramic article.

The magnesia source can, for example and without limitation, be selectedfrom one or more of MgO, Mg(OH)₂, MgCO₃, MgAl₂O₄, Mg₂SiO₄, MgSiO₃,MgTiO₃, Mg₂TiO₄, MgTi₂O₅, talc, and calcined talc. Alternatively, themagnesia source can be selected from one or more of forsterite, olivine,chlorite, or serpentine. Preferably, the magnesia source has a medianparticle diameter that does not exceed 35 μm, and preferably that doesnot exceed 30 μm. To this end, as referred to herein, all particlediameters are measured by a laser diffraction technique such as by aMicrotrac particle size analyzer.

The alumina source can, for example and without limitation, be selectedfrom an alumina-forming source such as corundum, Al(OH)₃, boehmite,diaspore, a transition alumina such as gamma-alumina or rho-alumina.Alternatively, the alumina source can be a compound of aluminum withanother metal oxide such as MgAl₂O₄, Al₂TiO₅, mullite, kaolin, calcinedkaolin, phyrophyllite, kyanite, etc. In one embodiment, the weightedaverage median particle size of the alumina sources is preferably in therange of from 10 μm to 60 μm, and more preferably in the range of from15 μm to 30 μm. In still another embodiment, the alumina source can be acombination of one or more alumina forming sources and one or morecompounds of aluminum with another metal oxide.

The titania source can, in addition to the compounds with magnesium oralumina described above, be provided as TiO₂ powder.

The silica source can be provided as a SiO₂ powder such as quartz,cryptocrystalline quartz, fused silica, diatomaceous silica, low-alkalizeolite, or colloidal silica. Additionally, the silica source can alsobe provided as a compound with magnesium and/or aluminum, including forexample, cordierite, chlorite, and the like. In still anotherembodiment, the median particle diameter of the silica source ispreferably at least 5 μm, more preferably at least 10 μm, and still morepreferably at least 20 μm.

As described above, one or more sintering aid(s) or additives canoptionally be added to the precursor batch composition to lower thefiring temperature and broaden the firing window required to form theceramic composition. The sintering aid can, for example, be present inan amount of from 0.15 to 5 weight percent of the total composition andcan include, for example, one or more of a metal oxide such as at leastone of CaO and SrO, or at least one of CaO and SrO in combination withone or more of CeO₂, Y₂O₃, and La₂O₃. The sintering aid(s) can be addedto the precursor batch composition as carbonates, silicates, aluminates,hydrates, etc. In one exemplary embodiment, calcium oxide (CaO) has beenfound to be a particularly good sintering additive when added in anamount of between about 0.15 and 1.0 wt %, and more preferably betweenabout 0.25 and 0.75 wt %. In one exemplary embodiment, strontium oxide(SrO) has been found to be a particularly good sintering additive whenadded in an amount of between about 0.1 and 2.0 wt %, and morepreferably between about 0.5 and 1.5 wt %. Similarly, an addition ofFe₂TiO₅ can be useful for slowing decomposition in oxidizing atmosphereswhen added in an amount of from 0 to 3 weight %.

Still further, the ceramic precursor batch composition may compriseother additives such as surfactants, oil lubricants and pore-formingmaterial. Non-limiting examples of surfactants that may be used asforming aids are C₈ to C₂₂ fatty acids, and/or their derivatives.Additional surfactant components that may be used with these fatty acidsare C₈ to C₂₂ fatty esters, C₈ to C₂₂ fatty alcohols, and combinationsof these. Exemplary surfactants are stearic, lauric, myristic, oleic,linoleic, palmitic acids, and their derivatives, tall oil, stearic acidin combination with ammonium lauryl sulfate, and combinations of all ofthese. In an illustrative embodiment, the surfactant is lauric acid,stearic acid, oleic acid, tall oil, and combinations of these. In someembodiments, the amount of surfactants is from about 0.25% by weight toabout 2% by weight.

Non-limiting examples of oil lubricants used as forming aids includelight mineral oil, corn oil, high molecular weight polybutenes, polyolesters, a blend of light mineral oil and wax emulsion, a blend ofparaffin wax in corn oil, and combinations of these. In someembodiments, the amount of oil lubricants is from about 1% by weight toabout 10% by weight. In an exemplary embodiment, the oil lubricants arepresent from about 3% by weight to about 6% by weight.

The precursor composition can, if desired, contain a pore-forming agentto tailor the porosity and pore size distribution in the fired body fora particular application. A pore former is a fugitive material whichevaporates or undergoes vaporization by combustion during drying orheating of the green body to obtain a desired, usually higher porosityand/or coarser median pore diameter. A suitable pore former can include,without limitation, carbon; graphite; starch; wood, shell, or nut flour;polymers such as polyethylene beads; waxes; and the like. When used, aparticulate pore former can have a median particle diameter in the rangeof from 10 μm to 70 μm, and more preferably from 15 μm to 50 μm.

The inorganic ceramic forming batch components, along with any optionalsintering aid and/or pore former, can be intimately blended with aliquid vehicle and forming aids which impart plastic formability andgreen strength to the raw materials when they are shaped into a body.When forming is done by extrusion, most typically a cellulose etherbinder such as methylcellulose, hydroxypropyl methylcellulose,methylcellulose derivatives, and/or any combinations thereof, serve as atemporary organic binder, and sodium stearate can serve as a lubricant.The relative amounts of forming aids can vary depending on factors suchas the nature and amounts of raw materials used, etc. For example, thetypical amounts of forming aids are about 2% to about 10% by weight ofmethyl cellulose, and preferably about 3% to about 6% by weight, andabout 0.5% to about 1% by weight sodium stearate, stearic acid, oleicacid or tall oil, and preferably about 0.6% by weight. The raw materialsand the forming aids are typically mixed together in dry form and thenmixed with water as the vehicle. The amount of water can vary from onebatch of materials to another and therefore is determined by pre-testingthe particular batch for extrudability.

The liquid vehicle component can vary depending on the type of materialused in order to impart optimum handling properties and compatibilitywith the other components in the ceramic batch mixture. Typically, theliquid vehicle content is usually in the range of from 15% to 50% byweight of the plasticized composition. In one embodiment, the liquidvehicle component can comprise water. In another embodiment, dependingon the component parts of the ceramic batch composition, it should beunderstood that organic solvents such as, for example, methanol,ethanol, or a mixture thereof can be used as the liquid vehicle.

Forming or shaping of the green body from the plasticized precursorcomposition may be done by, for example, typical ceramic fabricationtechniques, such as uniaxial or isostatic pressing, extrusion, slipcasting, and injection molding. Extrusion is preferred when the ceramicarticle is of a honeycomb geometry, such as for a catalytic converterflow-through substrate or a diesel particulate wall-flow filter. Theresulting green bodies can be optionally dried, and then fired in a gasor electric kiln or by microwave heating, under conditions effective toconvert the green body into a ceramic article. For example, the firingconditions effective to convert the green body into a ceramic articlecan comprise heating the green body at a maximum soak temperature in therange of from 1250° C. to 1450° C., for example, in the range of from1300° C. to 1350° C., or in the range of from 1330° C. to 1380° C., andmaintaining the maximum soak temperature for a hold time sufficient toconvert the green body into a ceramic article, followed by cooling at arate sufficient not to thermally shock the sintered article.

Still further, the effective firing conditions can comprise heating thegreen body at a first soak temperature in the range of from 1240 to1350° C. (preferably 1270 to 1330° C.), holding the first soaktemperature for a period of from 2 to 10 hours (preferably 4 to 8hours), then heating the body at a second soak temperature in the rangeof from 1270 to 1450° C. (preferably 1300-1350° C.), and holding thesecond soak temperature for a period of from 2 to 10 hours (preferably 4to 8 hours), again followed by cooling at a rate sufficient not tothermally shock the sintered article.

To obtain a wall-flow filter, a portion of the cells of the honeycombstructure at the inlet end or face are plugged, as known in the art. Theplugging is only at the ends of the cells which is typically to a depthof about 1 to 20 mm, although this can vary. A portion of the cells onthe outlet end but not corresponding to those on the inlet end areplugged. Therefore, each cell is plugged only at one end. The preferredarrangement is to have every other cell on a given face plugged in acheckered pattern.

A greater understanding of the findings underlying the presentdisclosure can be obtained with reference to phase equilibrium diagramsfor the MgO—Al₂O₃—TiO₂—SiO₂ system, prepared by at least one of thepresent inventors, and set forth in previously mentioned U.S. patentapplication Ser. No. 12/305,767. It will of course be recognized thatmany of the boundaries between phase fields included in such diagramsrepresent the results of equilibrium calculations and extrapolationrather than actual phase analyses. While the phase fields themselveshave been confirmed with experiments, the precise temperatures andcompositions representing boundaries between phase fields areapproximate. In any case, the phase diagram of FIG. 1 depicts the stablecombination of phases as a function of temperature and composition alongthe pseudo-binary join between aluminum titanate (Al₂TiO₅) andcordierite (Mg₂Al₄Si₅O₁₈). Essentially, this diagram indicates thatmixtures of cordierite and AT at high temperature will tend to formother phases, including mullite, titania, liquid, and a solid-solutionphase with the pseudobrookite crystal structure.

Two significant features can be derived from a study of this diagram.First, in order for the pseudobrookite phase to be in equilibrium withcordierite there is a general restriction on the composition of thesolid-solution, in particular, pure AT will tend to not exist inequilibrium with cordierite. FIGS. 2A and 2B calculated using Factsage™(by Thermfact and GTT-Technologies) depict the phase relations at 1325°C. in the ternary section with endpoints of magnesium dititanate,aluminum titanate, and cordierite within the quaternaryMgO—Al₂O₃—TiO₂—SiO₂ system, showing that the pseudobrookite phase PB inequilibrium with cordierite C contains at least about 25 wt % magnesiumdititanate at this temperature. FIG. 2A shows thecordierite-pseudobrookite phase diagram depicting the pseudobrookite PB,cordierite C, mullite M, sapphirine Sap, titania T, and liquid phaserelations at 1325° C. FIG. 2B shows the cordierite-pseudobrookite phasediagram with 10 wt % mullite depicting the pseudobrookite PB, cordieriteC, mullite M, sapphirine Sap, titania T, and liquid phase relations at1325° C.

Second, FIG. 1 shows that a liquid appears in the diagram at fairly lowtemperatures (˜1390° C., although the lowest eutectic liquid in thissystem exists well below this).

EXAMPLES

Exemplary embodiments of the claimed invention are further describedbelow with respect to certain exemplary and specific embodimentsthereof, which are illustrative only and not intended to be limiting. Inaccordance with some of the examples, a series of inventive ceramicarticles is prepared having the general inorganic batch composition asprovided in Table 1, in terms of the weight percentages of theend-member phases, and as provided in Table 2, expressed in terms of theweight percentages of the single component oxides, excluding anysintering additive.

TABLE 1 Formula Name Weight % Al₂TiO₅ AT 40 MgTi₂O₅ MT2 20 Al₆Si₂O₁₃Mullite 25 Mg₂Al₄Si₅O₁₈ Cordierite 15

TABLE 2 Metal Oxide Weight % MgO 6.10 Al₂O₃ 45.61 TiO₂ 33.54 SiO₂ 14.76

Tables 3 to 5 provide data for the composite aluminum titanate-magnesiumdititanate cordierite examples fabricated according to the generalcomposition of Tables 1 and 2. Listed are the raw materials, poreformers, and sintering aid (median particle diameters in parentheses)used to make the samples. The examples provided have been made bymulling component powders with water and an organic binder, followed byextrusion, drying, and firing. All extruded samples were wrapped in foiland hot-air dried. Samples were subsequently fired in an electric kilnby heating at 60° C./hr to a first soak temperature and holding for 6hours, then heated at 60° C./hr to a second soak temperature and heldfor another 6 hours. Soak temperatures are also provided in Tables 3 to5. These examples will be discussed further below. All measurements,except where noted, were on cellular pieces with 200 cells per squareinch and 406 μm (16 mil) wall thicknesses. All samples were fired in airin electric furnaces unless otherwise noted. CTE was measured parallelto the honeycomb channels by dilatometry. Porosity and pore sizedistribution were derived from mercury porosimetry measurements.

Also provided in Tables 3 to 5 is the “maximum ΔL at 1000° C.,” definedas the value of ΔL/L at 1000° C. due to thermal expansion upon heating athermal expansion specimen to 1000° C. from room temperature, minus theminimum value of ΔL/L that occurs during cooling of a thermal expansionspecimen from 1000° C. to that lower temperature at which the minimumvalue of ΔL/L exists. The values of maximum ΔL at 1000° C. are reportedin Tables 3 to 5 as a percentage value; thus, for example, a maximum ΔLat 1000° C. of 0.15% is equal to a ΔL value of 0.15×10⁻², which is alsoequivalent to 1500 ppm, or 1500×10⁻⁶ inches/inch. The value of maximumΔL at 1000° C. is a measure of the degree of hysteresis between thethermal expansion curves (ΔL/L vs. temperature) during heating andcooling.

In addition to measurement of the property data in Tables 3 to 5,several special measurements were made to characterize the thermalstability of the aluminum titanate-magnesium dititanate and cordieritecomposite materials, and to determine their pressure drop behavior whenused as a diesel particulate filter.

The thermal stability (decomposition rate) was assessed by two methods.In the first method, specimens of the aluminum titanate-magnesiumdititanate and cordierite composite materials and of a control aluminumtitanate composition were held at 1100° C. and their lengths monitoredover time for up to 100 hours. Decomposition of the pseudobrookite phaseis accompanied by a decrease in volume (shrinkage, or negative lengthchange). The results, shown in FIG. 3, demonstrate the superiorstability of the aluminum titanate-magnesium dititanate and cordieritecomposite materials, for which the decomposition rate of thepseudobrookite phase is at least 10 times slower than for the controlaluminum titanate composition. In a second method to assessdecomposition rate, the CTE of the aluminum titanate-magnesiumdititanate and cordierite composite materials and control aluminumtitanate composition was measured before and after isothermally holdingthe sample for 100 hours at temperatures of from 950 to 1250° C. Becausethe decomposition of the pseudobrookite phase reduces the amount ofmicrocracking, raising the CTE, the increase in CTE after heat treatmentis an indication of the degree of decomposition. The results are shownin FIG. 4, and demonstrate the improved thermal stability of thealuminum titanate-magnesium dititanate and cordierite composite bodies.

The pressure drops of clean and soot-loaded filters formed of arepresentative composite cordierite and aluminum titanate-magnesiumdititanate ceramic and an aluminum titanate control ceramic weremeasured on the bare and catalyzed filters. The filter of the compositecordierite and aluminum titanate-magnesium dititanate ceramic was of300/12 cell geometry. Washcoating was done after a conventionalpreliminary polymer solution passivation, using NYACOL® AL-20 colloidalalumina for the washcoat. Representative results of such pressure droptesting are set forth in FIG. 5, wherein the % increase in pressure dropafter washcoating is found to be lower for the filter of the compositecordierite and aluminum titanate-magnesium dititanate ceramic than forthe control aluminum titanate filter. The microstructure of thewashcoated filter thus tested is shown in FIG. 6.

The data in Tables 3 to 5 further illustrate some of the exemplaryranges in properties that can be achieved with the composite cordieriteand aluminum titanate-magnesium dititanate ceramic bodies of the currentclaimed invention. Examples 1 to 7 in Table 3 represent the baselinequaternary three-phase composition (Tables 1 and 2) with no sinteringadditive. These examples show that low thermal expansion (6 to 20×10⁻⁷/°C.) can be achieved with porosities (44-52%) and median pore diameters(15-27 μm) appropriate for application as a diesel particulate filter.The d_(f) values range from 0.24 to 0.45. The optimum top firingtemperature for these compositions is approximately 1355 to 1360° C. Thecoarser alumina used in Examples 4-7 results in higher pore size andlower firing shrinkage.

Examples 8 to 15 in Table 4 illustrate that the addition of about 2 wt.% Y₂O₃ to the base composition of Examples 1-3 allows a lower firingtemperature, between 1290-1320° C., and a broader range of firingtemperatures with high porosity (41-50%) and low thermal expansion (10to 14×10⁻⁷/° C.). Median pore diameters are 16 to 22 μm, and d_(f)values are reduced to 0.17 to 0.31. The change in shrinkage with firingtemperature is also lower. This allows a wider process window to achievethe desired properties. The optimum firing temperature is approximately1310° C.

Examples 16 to 22 in Table 5 demonstrate that the addition of only about1% Y₂O₃ super-addition to the base composition of Examples 1-3 reducesthe firing temperature to 1310-1350° C., with the optimum beingapproximately 1320° C. The lower level of additive results in a firingtemperature and firing process window that is intermediate between thebasic quaternary composition and that for 2 wt. % additive. Physicalproperties are still excellent for a diesel particulate filterapplication.

Examples 23-39 and 50-56 demonstrate that a sintering aid of ceriumoxide, mixtures of cerium oxide and yttrium oxide, mixtures of ceriumoxide, yttrium oxide, and lanthanum oxide, mixtures of cerium oxide andlanthanum oxide, or lanthanum oxide result in similar CTE, porosity,pore size, and pore size distribution at lower rare earth cost thanyttrium oxide alone.

TABLE 3 Example Number 1 2 3 4 5 6 7 Alumina A (24) 44.76 44.76 44.76 —— — — Alumina B (42) — — — 44.76 44.76 44.76 44.76 Alumina C (10) — — —— — — — Alumina D (18) — — — — — — — Silica A (25) — — — — — — — SilicaB (23) 8.65 8.65 8.65 8.65 8.65 8.65 8.65 Titania A (0.5) 33.85 33.8533.85 33.85 33.85 33.85 33.85 Magnesia A (1.2) 3.01 3.01 3.01 3.01 3.013.01 3.01 Talc A (5.0) 9.73 9.73 9.73 9.73 9.73 9.73 9.73 Talc B (14.4)— — — — — — — Talc C (23) — — — — — — — Y₂O₃ — — — — — — — Graphite A(35) 25.00 25.00 25.00 25.00 25.00 25.00 25.00 Corn Starch (17) — — — —— — — Potato Starch (49) — — — — — — — First Soak Temperature (° C.)1320 1330 1335 1325 1330 1335 1340 First Soak Time (hours) 6 6 6 6 6 6 6Second Soak Temperature (° C.) 1347 1357 1362 1352 1357 1362 1367 SecondSoak Time (hours) 6 6 6 6 6 6 6 Length Change after Firing (%) 1.7 −1.1−2.1 1.3 0.9 0.2 −0.5 CTE, 25-800° C. (10⁻⁷/° C.) 16.2 9.8 6.3 8.7 4.93.0 6.6 CTE, 25-1000° C. (10⁻⁷/° C.) 19.5 12.6 9.1 12.1 8.4 6.2 10.0Maximum ΔL at 1000° C. (%) 0.22 0.19 0.17 0.17 0.15 0.15 0.15 % Porosity52.1 50.6 44.0 52.1 51.5 51.5 44.6 d₅₀ (microns) 14.5 15.1 16.1 23.223.5 22.5 27.3 (d₅₀ − d₁₀)/d₅₀ 0.45 0.44 0.27 0.42 0.38 0.38 0.24 (d₉₀ −d₁₀)/d₅₀ 1.16 1.08 1.01 1.26 1.09 1.45 1.19

TABLE 4 Example Number 8 9 10 11 12 13 14 15 Alumina A (24) 43.90 43.9043.90 43.90 43.90 43.90 43.90 43.90 Alumina B (42) — — — — — — — —Alumina C (10) — — — — — — — — Alumina D (18) — — — — — — — — Silica A(25) — — — — — — — — Silica B (23) 8.48 8.48 8.48 8.48 8.48 8.48 8.488.48 Titania A (0.5) 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19Magnesia A (1.2) 2.96 2.96 2.96 2.96 2.96 2.96 2.96 2.96 Talc A (5.0)9.54 9.54 9.54 9.54 9.54 9.54 9.54 9.54 Talc B (14.4) — — — — — — — —Talc C (23) — — — — — — — — Y₂O₃ 1.94 1.94 1.94 1.94 1.94 1.94 1.94 1.94Graphite A (35) 30.00 30.00 30.00 30.00 30.00 30.00 30.00 30.00 CornStarch (17) — — — — — — — — Potato Starch (49) — — — — — — — — FirstSoak Temperature (° C.) 1275 1285 1290 1295 1305 1315 1320 1330 FirstSoak Time (hours) 6 6 6 6 6 6 6 6 Second Soak Temperature (° C.) 13021312 1317 1322 1332 1342 1347 1357 Second Soak Time (hours) 6 6 6 6 6 66 6 Length Change after Firing (%) −1.9 −2.8 −2.6 −3.5 −4.3 −4.6 −4.9−6.5 CTE, 25-800° C. (10⁻⁷/° C.) 6.8 7.4 6.3 7.4 7.5 11.2 9.6 8.3 CTE,25-1000° C. (10⁻⁷/° C.) 10.2 10.8 10.0 10.8 11.3 13.9 13.5 11.7 MaximumΔL at 1000° C. (%) 0.17 0.17 0.18 0.16 0.17 0.18 0.18 0.17 % Porosity50.4 48.3 49.3 47.2 45.7 43.9 41.5 41.1 d₅₀ (microns) 16.0 17.0 16.618.0 20.1 22.0 20.2 21.6 (d₅₀ − d₁₀)/d₅₀ 0.31 0.27 0.30 0.23 0.21 0.17 —0.17 (d₉₀ − d₁₀)/d₅₀ — — 0.75 0.60 0.71 0.79 — 0.87

TABLE 5 Example Number 16 17 18 19 20 21 22 Alumina A (24) 44.33 44.3344.33 44.33 44.33 44.33 44.33 Alumina B (42) — — — — — — — Alumina C(10) — — — — — — — Alumina D (18) — — — — — — — Silica A (25) — — — — —— — Silica B (23) 8.56 8.56 8.56 8.56 8.56 8.56 8.56 Titania A (0.5)33.52 33.52 33.52 33.52 33.52 33.52 33.52 Magnesia A (1.2) 2.99 2.992.99 2.99 2.99 2.99 2.99 Talc A (5.0) 9.63 9.63 9.63 9.63 9.63 9.63 9.63Talc B (14.4) — — — — — — — Talc C (23) — — — — — — — Y₂O₃ 0.98 0.980.98 0.98 0.98 0.98 0.98 Graphite A (35) 30.00 30.00 30.00 30.00 30.0030.00 30.00 Corn Starch (17) — — — — — — — Potato Starch (49) — — — — —— — First Soak Temperature (° C.) 1285 1290 1295 1305 1315 1320 1330First Soak Time (hours) 6 6 6 6 6 6 6 Second Soak Temperature (° C.)1312 1317 1322 1332 1342 1347 1357 Second Soak Time (hours) 6 6 6 6 6 66 Length Change after Firing (%) −0.9 −0.3 −1.1 −2.6 −3.7 −3.9 −5.1 CTE,25-800° C. (10⁻⁷/° C.) 11.3 11.6 8.4 8.4 7.2 6.3 10.8 CTE, 25-1000° C.(10⁻⁷/° C.) 14.6 15.3 11.8 11.7 10.9 9.7 14.3 Maximum ΔL at 1000° C. (%)0.19 0.20 0.17 0.18 0.17 0.18 0.18 % Porosity 51.3 51.9 50.5 51.1 43.943.9 42.5 d₅₀ (microns) 14.5 13.9 15.3 16.0 18.1 18.5 20.1 (d₅₀ −d₁₀)/d₅₀ 0.39 0.45 0.35 0.33 0.23 0.22 0.17 (d₉₀ − d₁₀)/d₅₀ 1.17 0.800.84 0.75 0.66 0.67 0.93

Examples 23-26 in Table 6 and Examples 41-49 in Table 13 include yttriumoxide as a sintering aid. Examples 27-30 in Table 6 and Examples 50-55in Table 13 include cerium oxide. Examples 31 and 32 in Table 6 containboth yttrium oxide and cerium oxide. Examples 38 and 39 in Table 10 andExample 56 in Table 13 include lanthanum oxide as a sintering aid.Example 40 in Table 10 includes no additional sintering aid. Theformulation of these examples is shown in Tables 6, 10, and 13. Examples23-32 all used 4% graphite and 22% starch (added as a superaddition tothe inorganic materials in Tables 6), and 4.5% methylcellulose and 1%tall oil added as superadditions to all the other batch components.These examples were mixed with deionized water, extruded into a cellularstructure with 300 cells per square inch and 330 μm (13 mil) wallthickness, dried and fired in gas-fired kilns to 1350° C. for 16 hours.The properties of the fired ware for examples 23-32 are shown in Table 6along with a relative cost estimate for the additive based on currentmarket prices normalized to the cost of 1% Y₂O₃.

Table 7 lists some representative prices of rare-earth materials, whichare at least a factor of 10 higher than all the other batch materials.

FIG. 7 shows the coefficient of thermal expansion (CTE) as a function ofrelative rare earth cost (1% Y₂O₃=1) for the comparative examples 23-26and the Examples 27-32 of Table 6. As FIG. 7 illustrates, the cost toattain a CTE below a given value, for example, below 12×10⁻⁷/° C. islower for cerium oxide or yttrium oxide and cerium oxide mixtures thanfor yttrium oxide alone while retaining similar pore size, porosity andpore size distribution (Table 6). The rare earth cost reduction possibleis at least 50% using this metric.

These lower cost compositions show similar stability of properties withfiring temperature as the higher-cost compositions. Table 8 shows theproperties of Examples 24, 25, 28, and 32 after firing for 12 hours at1320, 1330, 1340, 1350 and 1360° C. in an electric kiln. The porositywas measured by the Archimedes method (Arch Porosity).

TABLE 6 Example Number 23 24 25 26 27 28 29 Alumina A (24) — — — — — — —Alumina B (42) — — — — — — — Alumina C (10) 44.31 44.18 43.97 43.5444.18 43.97 43.54 Alumina D (18) — — — — — — — Silica A (25) — — — — — —— Silica B (23) 2.72 2.71 2.69 2.67 2.71 2.69 2.67 Titania A (0.5) 33.6233.52 33.36 33.03 33.52 33.36 33.03 Magnesia A (1.2) Talc A (5.0) 19.1619.10 19.01 18.82 19.10 19.01 18.82 Talc B (14.4) — — — — — — — Talc C(23) — — — — — — — Y₂O₃ 0.20 0.50 1.00 2.00 — — — CeO₂ — — — — 0.50 1.002.00 Graphite A (35) 4.0 4.0 4.0 4.0 4.0 4.0 4.0 Corn Starch (17) — — —— — — — Potato Starch (49) 22 22 22 22 22 22 22 First Soak Temperature(° C.) 1350 1350 1350 1350 1350 1350 1350 First Soak Time (hours) 16 1616 16 16 16 16 Length Change after Firing (%) −2.0 −2.7 −2.8 −3.7 −2.1−2.7 −3.7 CTE, 25-800° C. (10⁻⁷/° C.) 12.8 9.6 9.2 8.2 13.3 10.1 8.6CTE, 25-1000° C. (10⁻⁷/° C.) 15.8 12.5 11.8 10.8 16.4 13.1 11.6 %Porosity 55 53 52 52 55 53 53 d₅₀ (microns) 11 12 13 14 12 13 14 (d₅₀ −d₁₀)/d₅₀ 0.28 0.22 0.17 0.16 0.21 0.19 0.16 (d₉₀ − d₁₀)/d₅₀ 0.52 0.460.36 0.35 0.44 0.38 0.35 Relative RE Cost 0.20 0.50 1.00 2.00 0.13 0.250.50 Example Number 30 31 32 Alumina A (24) — — — Alumina B (42) — — —Alumina C (10) 42.72 43.97 43.97 Alumina D (18) — — — Silica A (25) — —— Silica B (23) 2.62 2.69 2.69 Titania A (0.5) 32.41 33.36 33.36Magnesia A (1.2) — — — Talc A (5.0) 18.47 19.01 19.01 Talc B (14.4) — —— Talc C (23) — — — Y₂O₃ — 0.75 0.25 CeO₂ 4.00 0.25 0.75 Graphite A (35)4.0 4.0 4.0 Corn Starch (17) — — — Potato Starch (49) 22 22 22 FirstSoak Temperature (° C.) 1350 1350 1350 First Soak Time (hours) 16 16 16Length Change after Firing (%) −4.5 −3.1 −3.2 CTE, 25-800° C. (10⁻⁷/°C.) 10.4 9.1 8.5 CTE, 25-1000° C. (10⁻⁷/° C.) 13.4 12.1 8.5 % Porosity50 54 53 d₅₀ (microns) 17 13 13 (d₅₀ − d₁₀)/d₅₀ 0.16 0.22 0.19 (d₉₀ −d₁₀)/d₅₀ 0.32 0.42 0.43 Relative RE Cost 1.00 0.81 0.44

TABLE 7 Cost in Mar, Cost in Dec, Metal 2012 ($/kg) 2011 ($/kg)Lanthanum Oxide ≥ 99.5% 19 35 Cerium Oxide ≥ 99.5% 16 30 Yttrium Oxide ≥99.99% 95 95

TABLE 8 Firing Firing Example Temperature Soak Arch CTE CTE LengthNumber (C.) time Porosity d50 df db 800 1000 Change 24 1320 12 54 120.21 0.47 14.6 17.7 −1.3 24 1330 12 55 12 0.22 0.48 12.4 15.7 −1.5 241340 12 55 13 0.20 0.44 13.4 16.8 −1.2 24 1350 12 54 14 0.16 0.41 12.915.9 −1.3 24 1360 12 50 15 0.15 0.42 12.4 15.4 −3.3 24 1320 12 54 130.16 0.39 11.7 14.9 −1.9 25 1330 12 54 13 0.18 0.37 10.9 14.0 −2.1 251340 12 54 14 0.16 0.36 11.7 14.6 −1.6 25 1350 12 51 15 0.15 0.39 12.015.1 −3.2 25 1360 12 48 16 0.14 0.43 13.0 16.2 −5.5 28 1320 12 54 130.18 0.36 16.3 19.4 −2.4 28 1330 12 55 14 0.17 0.37 14.7 18.0 −1.5 281340 12 54 15 0.15 0.35 14.2 17.5 −1.1 28 1350 12 54 15 0.20 0.39 15.118.0 −2.0 28 1360 12 49 16 0.23 0.48 13.3 16.4 −5.1 32 1320 12 54 130.17 0.35 13.4 16.8 −2.3 32 1330 12 54 14 0.14 0.34 13.1 16.3 −2.5 321340 12 54 14 0.15 0.36 12.0 15.2 −1.9 32 1350 12 52 15 0.17 0.40 12.615.8 −2.9 32 1360 12 47 15 0.18 0.43 13.3 16.5 −6.5

Examples 33-40 and 57-68 were made by dry-blending a large batch of thecomposition shown in Table 9 and adding the additions shown in Table 10and dry-blending again. The powder for each batch was pressed in a dieto form a 8×8×65 mm bar before firing. Tables 11 to 16 provide data forthe inventive examples fabricated according to the general compositionof Tables 9 and 10. The data parameters provided are as described abovefor Tables 3 to 5.

Examples 33 and 34 in Table 11 use cerium oxide as a sintering aid.Examples 35-39, shown in Table 11 use lanthanum oxide (La₂O₃) ormixtures of La₂O₃ with cerium oxide. Comparative example 40 in Table 10uses the batch composition with no sintering aid additive. Propertiesfor Examples 33-39 after firing at 1330° C. for 12 hours in an electrickiln are shown in Table 11. These results are similar to CeO₂ or Y₂O₃alone, but with an approximately 3×10⁻⁷/° C. higher thermal expansioncoefficient than with CeO₂ or Y₂O₃ alone.

Table 15 shows the properties as a function of firing temperature with a16 hour hold time for Examples 33 and 40 showing that CeO₂ provides awide firing window.

To further reduce cost, CaO, SrO and mixtures of CaO and/or SrO withCeO₂ can be used to achieve acceptable porosity, pore size distribution,CTE values, and firing window properties compared to yttrium oxideand/or yttrium oxide and a lanthanide oxide alone and at a lowerrelative rare earth cost compared to yttrium oxide and/or a lanthanideoxide alone. Properties of Examples 57-61 after firing in an electrickiln at 1330° C. for 12 hours are shown in Table 12. Properties ofExamples 69 to 72 are shown in Table 14. When compared with Example 33in Table 11, Examples 58-61 and 69 to 72 show that CaO can be added tothis family of compositions to achieve similar porosity, pore size, andpore size distribution as attained with CeO₂ or Y₂O₃, with 4-5×10⁻⁷/° C.higher thermal expansion coefficient (with less than a tenth of apercent of the cost for the additive).

Examples 62-66, shown in Table 10 use SrO or mixtures of SrO with ceriumoxide as a sintering aid. Properties for Examples 62-66 after firing at1330° C. for 12 hours in an electric kiln are shown in Table 12. Theseresults are similar to CaO as a sintering aid, but with a 5-7×10⁻⁷/° C.higher thermal expansion coefficient than with CeO₂ alone. However, thecost of using SrO is about 0.1% of the relative cost of using Y₂O₃.

Table 16 shows the properties of Examples 60, 61, 65, and 66 afterfiring at 1310, 1320, 1330, 1340, 1350, and 1360° C. for 16 hours. Table16 shows the properties as a function of firing temperature with a 16hour hold time for Examples 60, 61, 65, and 66 showing that CaO and SrOprovide a wide firing window.

In the exemplary embodiments of the composite aluminumtitanate-magnesium dititanate and cordierite described thus far, theaddition of CaO and SrO as a sintering aid compared with CeO₂ additionsappeared to result in a higher coefficient of thermal expansion (CTE).The inventors discovered that reheating compositions with CaO and SrOadditions after firing to temperatures below the original firingtemperature, but above a post-fire threshold temperature resulted in alower thermal expansion coefficient (CTE). The reduction in thermalexpansion coefficient observed is shown in Table 17. The Inventorsdiscovered the post-fire threshold temperature to be about 1000° C. andthat this temperature is time dependent. Table 17 shows a change in CTEfrom room temperature (RT) to 1000° C., where RT is about 23 to 25° C.,relative to the same composition fired to the same top temperature andtime (1330° C. for 12 hours), but cooled directly to RT at a constantrate of 200° C./hr for Examples 33, 61, and 65 after the indicatedchanges in heat treatments.

Further testing of exemplary embodiments of the claimed compositealuminum titanate-magnesium dititanate and cordierite composition wasconducted. The range of heat treatment to lower the CTE was furtherdefined by exploring a range of times and temperatures. FIG. 8 showsschematic exemplary embodiment of a time-temperature (t-T) graph 80illustrating top (first hold) temperature 82, low (second hold)temperature 84, and mid (third hold) temperature 86. The time at the top(first hold) temperature 82 is a first hold time t₁, the time at the low(second hold) temperature 84 is a second hold time t₂, and the time atthe mid (third hold) temperature 86 is a third hold time t₃. Accordingto the schematic exemplary embodiment shown in FIG. 8, the toptemperature 82 is at T₁, the low temperature 84 is at T₂, and the midtemperature 86 is at T₃. T₃ is greater than the threshold temperature(not shown) and less than T₁. The threshold temperature (not shown) isat a temperature less than T₁ and greater than T₂.

Table 18 shows the CTE data for Examples 33 and 61 using a range of lowand mid temperatures 84 and 86 and times (t). Table 19 shows theproperties after firing to 1320° C. for 16 hours and the CTE afterreheating to 1250° C./hrs for Examples 34, 60, 61, 67, and 68. Thetime-temperature data of Tables 17, 18, and 19 show that both time andthe low temperature 84 have an impact on the final CTE. The compositealuminum titanate-magnesium dititanate and cordierite materialcompositions of the exemplary embodiments should be cooled below about400° C. to gain a substantial impact of the post-fire heat treatment.The time-temperature data of Tables 17 and 18 also shows that times aslow as 2 hours at the mid temperature 86 can reduce the CTEsignificantly from the as-fired value.

For example, the low temperature 84 may be in a range of 25° C. to 500°C., the time t₂ at the low temperature 84 may be in a range of 1 hour to48 hours, the mid temperature 86 may be in a range of 850° C. to 1350°C., and the time t₃ at the mid temperature 86 may be in a range of 1hour to 24 hours. The top temperature may be from 1250° C. to 1450° C.,for example, 1330° C. to 1380° C., and the soak time t₁ may be in arange from 2 to 24 hours.

Table 20 includes analyzed phases and pseudobrookite composition in wt %as determined by X-Ray Diffraction (XRD) for Examples 53, 54, 57, and58. These analyzed examples were all fired at 1365° C. Thepseudobrookite composition was determined by the lattice parameters ofthe pseudobrookite phase as determined by XRD. The phase distributionwas determined by Rietveld refinement of the XRD pattern.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

TABLE 9 Alumina C (10) 44.42 Silica A (25) 2.73 Titania A (0.5) 33.66Talc C (23) 19.19 Pea Starch (27) 19.00 Graphite A (35) 8.00

TABLE 10 Baseline CaCO₃ SrCO₃ CeO₂ La₂O₃ Example Batch (%) (%) (%) (%)(%) 33  99.228 — — 0.772 — 34  98.847 — — 1.153 — 35  98.847 — — 0.8650.288 36  98.847 — — 0.577 0.577 37  98.847 — — 0.288 0.865 38  98.847 —— — 1.153 39  99.420 — — — 0.580 40 100.000 — — — — 57  99.017 0.116 —0.867 — 58  99.189 0.232 — 0.579 — 59  99.361 0.349 — 0.290 — 60  99.5430.466 — — — 61  99.689 0.311 — — — 62  98.963 — 0.171 0.866 — 63  99.079— 0.343 0.578 — 64  99.196 — 0.515 0.289 — 65  99.313 — 0.687 — — 66 99.541 — 0.459 — — 67  99.302 0.698 — — — 68  99.071 0.929 — — —

TABLE 11 Example Number 33 34 35 36 37 38 39 CeO₂ 1.00 1.50 1.13 0.750.38 0.00 0.00 La₂O₃ 0.00 0.00 0.38 0.75 1.13 1.50 0.75 Length Change0.03 −0.26 — — — — — after Firing (%) CTE, 25-800° C. 13 11 — — — — —(10⁻⁷/° C.) CTE, 25- 16 14 16 18 17 18 21 1000° C. (10⁻⁷° C.) % Porosity50 50 50 51 50 51 51 d₅₀ (microns) 13 13 13 13 13 13 12 (d₅₀ − d₁₀)/d₅₀0.23 0.17 0.20 0.23 0.20 0.23 0.28 (d₉₀ − d₁₀)/d₅₀ 0.37 0.29 Relative RECost 0.250 0.375

TABLE 12 Example Number 57 58 59 60 61 CeO₂ 1.13 0.75 0.38 — — CaO 0.080.17 0.25 0.23 0.34 SrO — — — — — Length Change −0.07 0.04 −0.90 0.11−0.14 after Firing (A) CTE, 25-800° C. 12 13 16 17 16 (10⁻⁷/° C. ) CTE,25-1000° C. 15 17 19 20 20 (10⁻⁷/° C. ) % Porosity 50 50 48 50 49 d₅₀(microns) 13 13 14 13 13 (d₅₀-d₁₀)/d₅₀ 0.22 0.21 0.22 0.28 0.22(d₉₀-d₁₀)/d₅₀ 0.35 0.30 0.36 0.44 0.40 Relative RE Cost 0.282 0.1890.096 0.002 0.003 Example Number 62 63 64 65 66 CeO₂ 1.13 0.75 0.38 — —CaO — — — — — SrO 0.16 0.31 0.47 0.62 0.42 Length Change — — — — — afterFiring (%) CTE, 25-800° C. — — — — — (10⁻⁷/° C. ) CTE, 25-1000° C. 17 1418 23 22 (10⁻⁷/° C. ) % Porosity 50 51 51 50 51 d₅₀ (microns) 13 14 1414 14 (d₅₀-d₁₀)/d₅₀ 0.23 0.22 0.24 0.25 0.25 (d₉₀-d₁₀)/d₅₀ — — —Relative RE Cost 0.282 0.190 0.097 0.004 0.003

TABLE 13 Example Number 41 42 43 44 45 46 47 48 Alumina D (18) 44.18 —44.18 44.18 44.18 — — — Alumina E (20) — — — — — 44.18 44.18 44.18Alumina C (10) — 44.18 — — — — — — Silica C (25) 2.71 2.17 2.71 2.712.71 2.71 2.71 2.71 Titania A (0.5) 33.52 33.52 33.52 33.52 33.52 33.5233.52 33.52 Talc B (14.4) 19.10 19.10 19.10 19.10 19.10 19.10 19.1019.10 Y₂O₃ 0.49 0.74 0.49 0.49 0.49 0.49 0.49 0.49 CeO₂ — — — — — — — —La₂O₃ — — — — — — — — Graphite A (35) 14 8 14 10 14 8 8 8 Potato Starch(49) 30 19 32 30 32 27 27 27 First Soak Temperature (° C.) 1351 13581349 1345 1351 1358 1355 1365 First Soak Time (hours) 16 16 16 16 16 1616 16 Length Change after Firing (%) −0.28 −1.11 −1.33 −1.55 −1.89 −2.02−2.5 −2.71 CTE, 25-800° C. (10⁻⁷/° C.) 12.9 8.1 11.7 9.6 13.25 10.8 9.711.7 CTE, 25-1000° C. (10⁻⁷/° C.) 16.1 11.3 14.9 13.2 16.3 13.9 14.114.4 % Porosity 63.3 55.41 63.5 59.85 61.65 57.18 56 57.64 d₅₀ (microns)16.24 13.14 17.6 17.15 17.16 19.63 18.93 17.81 (d₅₀ − d₁₀)/d₅₀ 0.21 0.220.35 0.41 0.63 0.24 0.23 0.16 (d₉₀ − d₁₀)/d₅₀ 0.53 0.44 0.82 0.78 0.760.70 0.54 0.39 Example Number 49 50 51 52 53 54 55 56 Alumina D (18)44.18 44.18 44.18 44.05 43.97 43.97 43.97 44.18 Alumina E (20) — — — — —— — — Alumina C (10) — — — — — — — — Silica C (25) 2.71 2.71 2.71 2.72.70 2.70 2.70 2.71 Titania A (0.5) 33.52 33.52 33.49 33.42 33.36 33.3633.36 33.52 Talc B (14.4) 19.10 19.10 19.08 19.05 19.01 19.01 19.0119.10 Y₂O₃ 0.49 — — — — — — — CeO₂ — 0.96 0.59 0.78 0.975 0.98 0.98 —La₂O₃ — — — — — — — 0.96 Graphite A (35) 10 14 10 10 10 10 10 14 PotatoStarch (49) 30 32 30 30 30 30 30 32 First Soak Temperature (° C.) 13551351 1345 1345 1345 1345 1345 1346 First Soak Time (hours) 16 16 16 1616 16 16 16 Length Change after Firing (%) −2.72 −1.31 −1.39 −1.5 −1.51−1.81 −2.19 −1.77 CTE, 25-800° C. (10⁻⁷/° C.) — 11.7 12.1 11 11.6 10.410.7 14.7 CTE, 25-1000° C. (10⁻⁷/° C.) — 15 15.9 14.6 15.2 14 14.2 17.9% Porosity — 63.06 59.58 59.52 61 59.73 59.24 — d₅₀ (microns) — 18.917.12 17.45 15.91 17.66 17.8 — (d₅₀ − d₁₀)/d₅₀ — 0.28 0.35 0.36 0.180.32 0.33 — (d₉₀ − d₁₀)/d₅₀ — 0.71 0.70 0.70 0.41 0.68 0.65 —

TABLE 14 Example Number 69 70 71 72 Alumina D (18) 44.09 44.21 44.2144.06 Alumina E (20) — — — — Alumina C (10) — — — — Silica C (25) 2.702.71 2.71 2.70 Titania A (0.5) 33.45 33.54 33.54 33.43 Talc B (14.4)19.06 19.11 19.11 19.05 Y₂O₃ — — — — CeO₂ — — — 0.49 La₂O₃ — — — —Strontium Carbonate 0.7 — — 0.28 Calcium Carbonate — 0.42 0.421 —Graphite A (35) 10 10 10 10 Potato Starch (49) 30 30 30 30 First Soak1345 1345 1351 1345 Temperature (° C. ) First Soak Time 16 16 16 16(hours) Length Change after −1.33 −1.33 −1.54 −1.73 Firing (%) CTE,25-800° C. 18 15.7 16 14.1 (10⁻⁷/° C. ) CTE, 25-1000° C. 21.5 19.1 1917.8 (10⁻⁷/° C. ) % Porosity 58.67 58.11 61.03 59.00 d₅₀ (microns) 16.3715.29 18.17 16.95 (d₅₀-d₁₀)/d₅₀ 0.32 0.39 0.33 0.31 (d₉₀-d₁₀)/d₅₀ 0.710.75 0.82 0.68

TABLE 15 Porosity (%) d50 (microns) CTE 800 CTE 1000 Example Number 3333 33 33 40 1% 40 1% 40 1% 40 1% Temperature None CeO₂ None CeO₂ NoneCeO₂ None CeO₂ 1310 51 51 9 11 23.0 11.5 26 15 1320 52 52 9 12 21.4 11.225 15 1330 53 51 10 12 20.9 10.5 24 14 1340 54 51 10 12 18.7 10.5 22 141350 53 51 11 13 17.4 9.9 21 13 1360 54 — 11 — 15.1 6.9 18 — 1370 52 —13 — 12.6 16 — Firing Length dbreadth dfactor Change Example Number 3333 33 40 1% 40 1% 40 1% Temperature None CeO₂ None CeO₂ None CeO₂ 13100.66 0.39 0.49 0.29 −0.2 −0.2 1320 0.69 0.42 0.47 0.29 0.3 0.0 1330 0.650.38 0.48 0.26 0.9 −0.1 1340 0.61 0.37 0.44 0.25 1.4 −0.1 1350 0.56 0.320.39 0.21 1.5 −0.3 1360 0.48 0.38 0.32 — 1.4 — 1370 0.39 — 0.26 — 0.5 —

TABLE 16 Example Number 60 61 65 66 60 61 65 66 0.23% 0.34% 0.62% 0.42%0.23% 0.34% 0.62% 0.42% Temperature (° C.) CaO CaO SrO SrO CaO CaO SrOSrO Porosity (%) d50 (microns) 1310 52 52 51 52 11 12 11 11 1320 53 5252 53 11 12 12 11 1330 53 52 52 53 12 12 12 12 1340 52 52 51 53 12 13 1312 1350 51 52 51 51 13 14 14 13 CTE 800 CTE 1000 1310 19.4 17.9 22.521.5 23 21 26 25 1320 18.7 16.3 19.3 21.1 22 19 23 25 1330 17.6 15.817.1 19.5 21 19 21 23 1340 16.0 14.6 16.7 17.6 19 18 20 21 1350 14.313.7 14.9 16.4 18 17 18 20 1360 7.1 5.3 6.6 9.0 — — — — dbreadth dfactor1310 0.60 0.46 0.43 0.48 0.43 0.29 0.28 0.31 1320 0.52 0.44 0.42 0.480.32 0.26 0.23 0.30 1330 0.46 0.53 0.42 0.48 0.31 0.35 0.23 0.33 13400.49 0.39 0.38 0.37 0.33 0.23 0.21 0.23 1350 0.40 0.62 0.37 0.41 0.240.26 0.21 0.29 1360 0.49 0.49 0.40 0.44 — — — — Firing Length ChangeExample Number 60 61 65 66 0.23% 0.34% 0.62% 0.42% Temperature (° C.)CaO CaO SrO SrO 1310 0.6 0.3 −0.3 0.0 1320 1.2 0.4 0.2 0.6 1330 0.9 0.40.3 0.5 1340 0.9 0.4 0.2 0.7 1350 0.7 −0.1 −0.1 0.6

TABLE 17 Example 33 61 65 Additive 1% CeO₂ 0.34% CaO 0.62% SrO Hold1300° C. —8 hrs 0.3 −0.2 0.1 during cooling Hold 1275° C. —8 hrs −0.20.2 −1.7 during cooling Hold 1250° C. —8 hrs −0.6 −0.3 −0.9 duringcooling Reheat to 1300° C. —8 hours −0.7 −3.1 −3.4 after firing Reheatto 1275° C. —8 hours −0.5 −2.5 −4.6 after firing Reheat to 1250° C. —8hours −0.4 −3.5 −4.4 after firing

TABLE 18 Time at Mid Time at Low Low Tem- Temper- Mid Tem- CTE 1000° C.Temper- perature ature perature Example Example ature (hr) (hr) (hr) 3361 RT >24 None — 14.1 16.9 RT >24 1250  2 14.3 12.2 RT >24 1250  8 13.611.9 RT >24 1250 16 13.5 11.0 RT >24 1200  8 13.3 12.6 RT >24 1000  815.3 17.6  400° C. 2 1250  8 13.2 13.8  800° C. 2 1250  8 13.8 14.01000° C. 2 1250  8 13.8 14.4  400° C. 16 1250  8 14.1 14.5  800° C. 161250  8 13.4 14.6 1000° C. 16 1250  8 14.3 13.1

TABLE 19 CTE 1000° C. after reheating to CTE 1250° C./ Example CaO CeO₂Porosity 1000° C. d₅₀ d_(f) hrs 34 0.00 1.5 50.5 13.8 12.7 0.27 12.7 600.23 0.0 52.7 21.4 12.0 0.31 17.1 61 0.34 0.0 51.6 18.2 12.2 0.23 12.867 0.51 0.0 50.6 16.1 13.1 0.22 12.5 68 0.68 0.0 49.7 15.7 13.9 0.2512.4

TABLE 20 Example Al_(2(1−x))Mg_(x)Ti_((1+x))O₅ Number PseudobrookiteCorundum Cordierite Mullite Y₂Ti₂O₇ CeO₂ Value of x 25 65 3 16 16 0.40.0 0.19 26 65 1 13 20 1.3 0.0 0.19 29 67 1 14 18 0.0 0.0 0.19 30 67 110 20 0.0 1.5 0.19

The invention claimed is:
 1. A ceramic article, comprising: apseudobrookite phase comprising predominately alumina, magnesia, andtitania; a second phase comprising cordierite; and a sintering aid,comprising: (i) at least one of a calcium oxide and strontium oxide, and(ii) at least one of a yttrium oxide, a lanthanum oxide, and a ceriumoxide; wherein the ceramic article comprises a total porosity of atleast 40% by volume, a median pore size d₅₀ in a range of 10 μm to 30μm, and a composition expressed on an oxide basis ofa(Al₂TiO₅)+b(MgTi₂O₅)+c(2MgO.2Al₂O₃.5SiO₂)+d(3Al₂O₃.2SiO₂)+e(MgO.Al₂O₃)+f(2MgO.TiO₂)+g(CaO)+h(SrO)+i(X)+j(Fe₂O₃.TiO₂)+k(TiO₂)+1(Al₂O₃),wherein X is at least one of CeO₂, Y₂O₃, and La₂O₃, and a, b, c, d, e,f, g, h, i, j, k, and l are weight fractions of each component such that(a+b+c+d+e+f+g+h+i+j+k+1)=1.00, and wherein 0.3≤a≤0.75, 0.075≤b≤0.3,0.02≤c≤0.5, 0.0≤d≤0.4, 0.0≤e≤0.25, 0.0≤f≤0.1, 0.0≤g≤0.01, 0.0≤h≤0.02,0.0015≤(g+h), 0.0≤i≤0.05, 0.0≤j≤0.03, 0.0 k 0.2, and 0.0≤1≤0.1.
 2. Theceramic article of claim 1, wherein the sintering aid comprises a ceriumoxide.
 3. The ceramic article of claim 1, wherein the sintering aidcomprises cerium oxide and at least one of yttrium oxide and a lanthanumoxide.
 4. The ceramic article of claim 1, wherein the ceramic articlehas a composition comprising, as expressed in weight percent on an oxidebasis, from 4 to 10% MgO; from 40 to 55% Al₂O₃; from 25 to 44% TiO₂; andfrom 5 to 25% SiO₂.